1. Field of the Invention
The present invention relates to a photoelectric conversion device, a production method thereof, a photosensor, an imaging device and their driving methods.
2. Description of the Related Art
As for the solid-state imaging device, there is widely used a flat light-receiving device where photoelectric conversion sites are two-dimensionally arrayed in a semiconductor to form pixels and a signal generated by photoelectric conversion in each pixel is charge-transferred and read out through a CCD or CMOS circuit. The conventional photoelectric conversion site generally used is a photodiode part formed using PN junction in a semiconductor such as Si.
In recent years, fabrication of a multipixel device is proceeding, and the pixel size and in turn, the area of a photodiode part become small, which brings about problems of reduction in the aperture ratio, reduction in the light collection efficiency and the resulting reduction in the sensitivity. As for the measure to increase the aperture ratio and the like, suggestions are being made on a solid-state imaging device having an organic photoelectric conversion film using an organic material.
In the organic photoelectric conversion film, a technique of introducing a mixed film using a fullerene or a fullerene derivative or a bulk heterojunction structure into the organic photoelectric conversion film so as to bring out high photoelectric conversion efficiency (high exciton dissociation efficiency) is known. For example, Patent Document 1 discloses a photoelectric conversion film containing a fullerene or a fullerene derivative.
The organic photoelectric conversion device used in a solar cell is designed to collect electric power and therefore, an external electric field is not applied, but the organic photoelectric conversion device used as a visible light sensor of a solid-state imaging device needs to maximize the photoelectric conversion efficiency and a voltage is externally applied so as to enhance the photoelectric conversion efficiency or increase the response speed.
When a voltage is externally applied so as to enhance the photoelectric conversion efficiency or increase the response speed, injection of a hole or injection of an electron from an electrode is generated due to an external electric field, and this disadvantageously increases the dark current, which becomes a problem.
Many of materials usually used as an electrode in a photoelectric conversion device have a work function (WF) of around 4.5 eV (for example, ITO) and, for example, in the case of using a fullerene (C60) as the material of the photoelectric conversion film, an energy gap between WF of the electrode and LUMO of the fullerene (C60) becomes small, as a result, particularly an electron is liable to be injected from the electrode into the photoelectric conversion film and a significant increase of dark current is caused.
As regards the prevention of an increase in the dark current due to an injected current, a technique of providing a charge blocking layer to suppress the injection of an electric charge into the photoelectric conversion layer, thereby efficiently blocking an injected carrier and reducing the dark current, is disclosed (Patent Document 2).
In the case of using the photoelectric conversion device as a solid-state imaging device, a color filter needs to be provided for the formation of a color image and furthermore, when soldering the imaging device to a substrate, the device with the substrate is heated. Accordingly, it is demanded that the reduction in photoelectric conversion efficiency and the increase of dark current are small under the conditions of a temperature not less than 180° C. which is the process temperature in the above, and about 30 minutes. However, in Patent Documents 1 and 2, heat resistance working out to an important factor in practice is not referred to, and a chemical structure having high heat resistance is not sufficiently described.
In Patent Documents 3 to 6, an organic material having a fluorene structure is described, and the organic material is used for an electroluminescence device. The electroluminescence device utilizes luminescence occurring upon application of a voltage to the device, but the photoelectric conversion device when emitted light is reduced in the photoelectric conversion efficiency and therefore, is required to emit substantially no light. Also, the transfer direction of a hole is opposite between the electroluminescence device and the photoelectric conversion device and in turn, the functions required of the material differs.
The photoelectric conversion device outputs signals according to the quantity of light entered, and a constant voltage therefor is applied to the device. As described above, an important function of the electron blocking material in the photoelectric conversion device is suppression of electron injection from the electrode. On the other hand, in the electroluminescence device, the light/dark luminescence is controlled by a voltage, and the above-described function is unnecessary.
In Patent Document 7, an organic material having a fluorene structure is described, and the organic material is used for a dye-sensitized solar cell. However, the characteristics required of a solar cell differ from those required of the photoelectric conversion device aiming at an imaging device element and therefore, unlike the present invention, description regarding dark current and heat resistance is not sufficiently disclosed.
Also, in the case of producing a film by using the compound described in Patent Document 7, generation of a grain boundary by crystallization due to low amorphous property and formation of unevenness on the film surface may be caused, and this material is not suited as a material of a photoelectric conversion device aiming at a photosensor, an imaging device and the like.
[Patent Document 1] JP-A-2007-123707 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)
[Patent Document 2] JP-A-2008-72090
[Patent Document 3] JP-A-2005-290000
[Patent Document 4] Japanese Patent 3,508,984
[Patent Document 5] JP-A-2007-137795
[Patent Document 6] JP-A-2006-131783
[Patent Document 7] JP-A-2007-115665